Geomagnetic Storms as a Dominant Component of Space Weather: Classic Picture and Recent Issues

Abstract

It was in the mid-1800s that extraordinary, worldwide disturbances in the Earth's magnetic field were coined “geomagnetic storms.” It would not be too much of an exaggeration to state that space weather predictions originated in early studies of geomagnetic storms. The effects of geomagnetic storms in space surrounding Earth result from a chain of processes involving flow/transformation of solar wind energy, and electrodynamic coupling among the interplanetary medium, magnetosphere, ionosphere, and upper atmosphere. The importance of predicting geomagnetic storms lies not only in its “academic” purposes in understanding physical processes in the solarterrestrial environment, but also in its practical aspects, influencing societal problems such as the effects on communications and satellite anomalies. This chapter discusses the characteristic signatures of geomagnetic storms, obtained from a number of statistical studies, and addresses recent major issues which impact directly our fundamental understanding of solar wind effects on magnetospheric and ionospheric processes, i.e., solar wind control of geomagnetic storms, storm/substorm relationships, ring current constituents, and solar cycle and seasonal dependence of geomagnetic storms. The following are the main points of the discussion: (1) Most of the Dst variance during intense geomagnetic storms can be reproduced by knowledge about changes in large-scale electric fields in the solar wind. A continuing controversy exists, however, as to whether the successive occurrence of substorms plays a direct role in the energization of storm-time ring current particles. (2) The increase in the ring current of about 50% of the largest geomagnetic storms goes through two steps at the main phase. The solar wind causes of this double enhancement in the ring current must be identified. (3) CMEs (coronal mass ejections) and CIRs (corotating interaction regions) appear to be the primary sources leading to major geomagnetic storms. These are dominant near the maximum phase and during the declining phase of the solar cycle, respectively. The 22-year solar cycle dependence of geomagnetic activity must also be quantitatively evaluated in terms of CMEs and CIRs. (4) Recent satellite observations in the inner magnetosphere have shown that the abundance of ionospheric origin ions is high and is correlated well with substorm activity during the main phase of geomagnetic storms. The relative importance of solar wind-origin and ionosphere-origin ions in constituting ring current particles is currently a critical unsolved question.